Three phase differential mode harmonics are typically filtered by placing three inductors in series with the line between the drive and motor. Common-mode harmonics are typically filtered by placing three parallel conductors on one magnetic core path.
With relation to three phase AC motor controllers, particularly pulse width modulation (PWM) voltage source inverters (VSI), each phase of the three phases of a motor is connected to a VSI by a separate conductor. PWM VSI's operate by switching a DC voltage at a high frequency. All multiple conductor wire runs contain stray inductance and stray capacitance. This creates the possibility of a series resonant circuit in the motor cable system. The longer the motor cables, the lower the resonant frequency. The output of a PWM VSI Drive contains switching frequencies that can excite this natural resonance. If the switching frequency of the output power devices is high enough, and if the resonant frequency of the motor cable system is low enough, voltage spikes at the AC Motor terminals can easily reach double the DC bus voltage. These elevated voltages can cause premature failure of motors or damage the cables supplying the motor.